Recent discoveries of (a) glasses for use in LTCC thick film materials which LTCC thick film materials achieve a sufficiently low dielectric constant (k) to allow for use as the low k portion of an electronic package for signal processing applications, and (b) such LTCC thick film materials which achieve a sufficiently low k to allow for use as the low k portion of an electronic package for signal processing applications, have created the possibility for additional discoveries utilizing such LTCC thick film materials. Such glasses are disclosed and claimed in commonly-assigned, allowed U.S. patent application Ser. No. 11/543,742, filed Oct. 5, 2006. Such low k thick film dielectric compositions are disclosed and claimed in commonly-assigned, U.S. patent application Ser. No. 11/824,116, filed Jun. 29, 2007.
An interconnect circuit board is a physical realization of electronic circuits or subsystems made from a number of extremely small circuit elements that are electrically and mechanically interconnected. It is frequently desirable to combine these diverse type electronic components in an arrangement so that they can be physically isolated and mounted adjacent to one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways, called vias, through a dielectric layer. Such a multilayer structure allows a circuit to be more compact.
A useful dielectric tape composition is disclosed in U.S. Pat. No. 6,147,019 to Donohue et al. The Donohue et al. dielectric tape composition achieves a dielectric constant in the range of 7-8 and is not suitable as a low k material for electronic packaging signal processing applications.
A further useful dielectric tape composition is commercially available Product No. 951 (commercially available from E.I. du Pont de Nemours and Company). This dielectric tape composition achieves a dielectric constant in the range of 7-8 and is not suitable as a low k material for electronic packaging signal processing applications.
Previously available LTCC thick film materials have not achieved a sufficiently low k to allow for use as the low k portion of an electronic package for signal processing applications. A typical use of thick film dielectric layers with a dielectric constant (k) of greater than 6 is in buried passive component applications. In these LTCC buried passive component applications, dielectric thick films are common.
However, in antenna, beamforming, filters, couplers, baluns, and other Radio Frequency (RF) signal processing applications it is preferable to use lower k materials having a k value substantially below 7-8, preferably low k materials having dielectric constants in the range of 2 to 5. Accordingly, the materials that have been used in those applications are not LTCC materials, rather they are poly-tetra-fluoro-ethylene (PTFE) materials, such as Teflon® commercially available from E.I. du Pont de Nemours and Company.
These PTFE materials can achieve a dielectric constant (k) of approximately 2.2-4. This dielectric constant of 3-4 allows for a wider line width and creates the ability to maintain 50 ohms and to achieve lower dielectric loss of the circuit and lower tolerance effects from the screen patterning the lines. Today, low k PTFE dielectrics are used in nearly all RF modules above 30 GHz due to wavelengths in the dielectric media being smaller.
The dielectric constant k is also an important material property that controls electrical performance of laminate materials. Dielectric laminate materials used in electronic industry generally fall in to two broad categories based on their dielectric constants, 1) low k materials having dielectric constants in the range of 2.2 to 5.5, and 2) high k materials with k ranging from 5.5 to 12.
Low k laminates are generally composed of complex organic materials such as the PTFE materials discussed above and high k materials are of glass-ceramic compositions.
Organic laminates may or may not have glass, ceramic, or other material particles loaded in their molecular matrix to realize different k values. LTCC materials are one of the most widely used glass ceramic material systems. Both low k and high k materials systems offer certain unique advantages owing to their dielectric constants. A comparison of specific advantages of each material group is shown in table 1.1 below.
Ceramic materials in general and LTCC materials specifically offer certain very attractive properties for electronic system level packaging, namely, (i) high mechanical strength and rigidity, (ii) hermeticity, (iii) extremely low water absorption (which is a serious issue with organic materials), (iv) excellent CTE (Coefficient of Thermal Expansion) match to industry standard semiconductor materials used in the manufacturing of integrated circuit (IC) chips.
Semiconductor materials have CTE values in the range of 3 to 5 ppm/° C., LTCC materials have CTE values close to 4.5 ppm/° C. compared to values of 17 ppm/° C. for organic materials. The close CTE match of LTCC values to that of IC chips results in very high reliability.
Furthermore, Ceramic materials in general and LTCC materials specifically (v) have smaller levels of cross talk due to higher dielectric constant. Finally, (vi) the higher dielectric constant k of LTCC materials results in very compact circuit designs.
However, the high dielectric constant k of LTCC materials makes it difficult to fabricate antennas and other radiating elements, especially at higher frequencies.
Another drawback of LTCC materials is that their higher dielectric constants k reduce signal transmission speeds.
Based on above discussion and Table 1.1 below it is clear that both low k and high k dielectric constant materials have specific advantages and disadvantages.
U.S. Pat. No. 5,757,611 to Gurkovich et al. discloses an electronic package having a buried passive component such as a capacitor therein, and a method for fabricating the same. The electronic package includes a passive component portion which includes a plurality of layers of high k dielectric material, a signal processing portion which includes a plurality of layers of low k dielectric material, and at least one buffer layer interposed between the passive component portion and the signal processing portion. Gurkovich et al. does not disclose an LTCC structure which allows for the absence of a buffer layer between the low k and high k regions. Furthermore, Gurkovich et al. discloses a method of fabrication which utilizes pressure assisted lamination. Gurkovich et al. discloses the use of passive component portions in conjunction with signal processing and does not disclose the ability for passive component portions and signal processing as stand-alone features. Additionally, Gurkovic et al. discloses the use of capture pads along all vertical vias between all layers.
Multilayer laminate structures that have both low k and high k layers have also been proposed that combine the advantages of both classes of laminate materials while limiting any adverse impact on circuit performance. However, their production methods are unwieldy and complex and the individual low k and high k components are expensive, both of which give rise to a high cost per unit manufactured. This will be described in more detail below.
The inventors have discovered that a number of circuit types could benefit from the combination of the positive aspects of both low k and high k materials in a simple, easy to manufacture LTCC laminate structure. For example, transceiver (combination of receiver and transmitter in the same circuit) circuits are a good example of such systems. Transceiver circuits are very widely used in a variety of communication equipment like cell phones, wireless network gear, radar systems, radios, etc.
A transceiver typically has a number of microwave integrated circuits (MMIC) made of semiconductors, passive components (as separate components and printed on the circuit substrate), and antennas for receiving and transmitting signals. For the packaging of electronic systems which use transceivers, the substrate material needs to be able to support attachment of MMIC, have the ability to support electrical interconnects with high routing density but lower cross talk, and be able to support efficient antenna structures.
There is a need for a simple to manufacture, inexpensive substrate material for transceivers, or other circuits such as receivers and transmitters, which combines the properties of both low k and high k materials.
The invention provides unique and novel methods and resulting structures in which two LTCC tape systems with differing dielectric constants (one with low k and the other with high k) can be combined to form a single, monolithic, multilayer circuit board for transceiver applications, or other circuits applications such as receivers and transmitters.
TABLE 1.1Comparison of Properties of Low k and High k MaterialsTypeAdvantagesDisadvantagesHigh-KTighter confinement of fields to theTighter field confinement makes it impractical tosubstrate resulting in lower cross-talk andfabricate antennas on high-K materials (withless unwanted radiation (EMI/EMC).K > 4). This means there needs to be a hybridLower radiation lossessolution (more than one dielectric material) forHigher routing density due to closertransmitters and receivers which require both chipspacing of interconnectsattach layers and antennas. Leads to significantAt any given frequency, the package sizeissues in processing/assembly/sourcing, andreduces by approximately the square rootcost.of dielectric constant. At relatively lowerReduction in physical dimensions of packagesfrequencies (<20 GHz) this will lead toand interconnects can impose very stringent andsmaller package sizesoften impractical fabrication tolerances atmillimeter wave frequencies (>40 GHz). This mayalso have a cost impact since costly photo-imaging patterning processes need to be used.Transmission losses can be higher for high Kmaterials due to two reasons. 1) frequencydependent attenuation constant increases withsquare root of dielectric constant, 2) Narrowerlines are needed for a given thickness andimpedance for high-K materials leading to higherohmic losses in conductors.Substrate needs to be thicker for a givenimpedance and line width for high-k materials.This will lead to larger aspect ratios for via andhence longer via resulting in higher parasiticinductanceCut-off frequency of higher order modes (which isa significant limiting factor in useable frequencyband for interconnects) decreases with higher KSlower velocity of propagation for signals sincevelocity reduces by a factor of 1/sqrt(K).Higher dispersion leading to detrimental effects insignal integrityLow-KPossible to fabricate antennas and otherHigher cross-talk and EMI, if not designed withradiating structures as well as function asproper diligence.a chip attach substrate. This means singleRequire larger spacing between traces leading tomaterial solution for transmitters andlower routing density. However, routing density isreceivers, resulting in relatively simplerusually not a liming factor for microwave circuits.assembly and lower cost.Traces and antenna patches are larger insize at any given frequency (compared tohigh-K materials) resulting in less stringentfabrication tolerances. Has significant costimpact since lower cost patterningtechnologies can be used.Generally low transmission lossRequire thinner dielectric thickness toachieve a given impedance for a giventrace width. This results in moremanufacturable aspect ratios for vias andshorter via lengths (lower parasiticinductance.)Cut off frequencies for interconnects andcircuits are higher leading to viableoperation beyond 100 GHz.